In wireless communication networks, efficient utilization of the media is important for low and high data rates. For low data rate networks, such as sensor networks, efficient transmission scheduling decreases power consumption and cost. For high data rate networks, such as audio-video (AV) networks, efficient transmission scheduling increases data throughput and improves quality of service (QoS).
Because signals transmitted by all transceivers, commonly called nodes, share the same frequency channel, it is necessary to enforce a channel access protocol to efficiently utilize the available network bandwidth. This can be done with a channel access schedule, which determines when and how nodes can access the shared channel.
For synchronization and network management, communication networks can partition time into periodic time intervals, when superframes are transmitted. A superframe can be further partitioned into a beacon period, a data transmission period, and in some cases, an idle period. Generally, the duration of the superframe is designed to guarantee satisfactory network synchronization, and to minimize transmission latency.
Data Transmission
The data transmission period can be partitioned into a contention-free period (CFP), and a contention-access period (CAP). During the CFP, a node first reserves or is allocated a timeslot to transmit data. During the CAP, nodes use some contention technique, such as carrier sense-multi access/collision detection (CSMA/CD) to transmit data.
Beacon Period
During the beacon period, network parameters are transmitted, i.e., transmission rates, logical channels, network identifiers, and the channel access schedule. The beacon period also defines the start of the CFP, the start of the CAP, and in some cases, the access schedule for the CFP. The beacon period can also include other parameters as defined by an applicable standard.
In general, for both the beacon period and the CFP, time is further partitioned into allocatable timeslots. In the case of the beacon period, the timeslot is referred as a beacon period slot. In the case of CFP, the timeslot is referred to as a guaranteed timeslot (GTS) or reserved timeslot. The CAP can also include timeslots, which are not allocated to any particular node. The number of timeslots in the beacon period, CFP and CAP can vary.
During a conventional timeslot for the beacon period or the CFP, only the node that has been allocated to the timeslot is allowed to transmit data. Even if the duration of the transmission is only a fraction of the timeslot or the node does not transmit in every superframe, all other nodes are not allowed to use the same timeslot in any superframe. Therefore, the length of the timeslot is an important design consideration in conventional networks. If the timeslot is too short, then overhead can increase. If the timeslot is too long, then throughput can be reduced.
All nodes in the network are allowed to access timeslots in the CAP by using the contention access mechanisms defined for the network. However, each timeslot in the beacon and the CFP is allocated to one specific node, and all other nodes are not allowed to access the network during that time in any superframe.
With a fixed length of the timeslots, the total numbers of slots for the beacon period and CFP are limited. For example, the IEEE 802.15.3 standard (WiMedia) specifies a maximum of forty-eight beacon period slots. The IEEE 802.15.4 standard (ZigBee) specifies a maximum of seven GTS in each superframe. Both of these standards promote interoperability of networks on a worldwide basis.
Such conventional timeslot allocation schemes impose the following limitations on the wireless network. The allocated timeslot is not fully utilized when the node has nothing to transmit. The total number of nodes that have access during the beacon period or CFP is limited by the number of timeslots. The conventional timeslot allocation scheme also has implications on power consumption, as in many networks nodes are required to transmit their own beacon, and ‘listen’ to other beacons in the channel in every superframe.